Performance study of diffusive gradients in thin films (DGT) for 55 elements

Abstract

The technique of diffusive gradients in thin films (DGT) is a fairly new and useful tool for in situ measurement of labile metal ions in water. In this study the applicability of DGT for determining 55 elements has been investigated by comparing independently determined or estimated diffusion coefficients with DGT effective diffusion coefficients (DDGT) calculated from the measured accumulation of mass by the DGT sampler. An exposure chamber was developed, where DGT could be exposed at controlled water fluid velocity. By this setup we determined DDGT-values from DGT samplers exposed at controlled concentration and fluid velocity at 4 pH levels between 4.7 and 6.0. Our DDGT values for the elements Co, Ni, Cu, Zn, Cd, Pb, Al, Mn and Ga were close to previously published values with some deviations for Pb and Zn. The uptake of V, Cr, Fe, U, Mo, Ti, Ba and Sr varied with pH and we recorded some experimental problems that require further investigations. We established a novel set of DDGT values for the lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Yb, Lu and Y) that all are very consistent. The DDGT values are about 10-15% lower than what can be calculated for free ions in water, and indicate that diffusion coefficients of metal ions in the agarose polyacrylamide hydrogel are 10-15% lower than in water. The high consistency of the data for the lanthanides establishes these elements as new performance test metals for the DGT sampler. The accumulation of the elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, Th and Ag was small (DDGT lower than 10% of theoretical values). Furthermore we established a more efficient elution procedure using concentrated nitric acid for the absorbent gel, with elution efficiencies between 95-100 % for most metals. For deployment times of 24 hours, detection limits from 0.001-1 ng/ml were readily achieved with moderate precautions to prevent contamination.